1. Field of the Invention
The present invention relates in general to cathodic materials for use in fuel cells and electrochemical sensors and, more particularly, to carbonaceous materials associated with oxygen reduction catalysts, for example, substituted and unsubstituted transition metal porphyrins, substituted and unsubstituted transition metal tetrabenzoporphyrins, substituted and unsubstituted transition metal tetraphenylporphyrins, substituted and unsubstituted transition metal tetraazaporphyrins, substituted and unsubstituted transition metal tetraazamacrocycles, substituted and unsubstituted transition metal phthalocyanines, substituted and unsubstituted transition metal naphthalocyanines, substituted and unsubstituted transition metal bis(phthalocyanines), substituted and unsubstituted transition metal bis(naphthalocyanines), and combinations thereof. The present invention further relates to fuel cells and electrochemical sensors having novel structural configurations, wherein the anode and the cathode of an associated micro electrode assembly (MEA) are disposed upon the same side or different sides of an ion exchange membrane, and/or exposed to the same gaseous environment or different gaseous environments—among other configurations. The present invention also relates to gas, smoke and/or fire detectors comprising novel electrochemical sensors, micro electrode assemblies, and/or sub-components for the same, as well as associated methods of manufacturing.
2. Background Art
Fuel cells and electrochemical sensors for use in, for example, gas, smoke, and/or fire detectors have been known in the art for several years. See, for example, U.S. Pat. No. 4,329,214 entitled “Gas Detection Unit,” U.S. Pat. No. 5,302,274 entitled “Electrochemical Gas Sensor Cells Using Three Dimensional Sensing Electrodes,” U.S. Pat. No. 5,331,310 entitled “Amperometric Carbon Monoxide Sensor Module for Residential Alarms,” U.S. Pat. No. 5,573,648 entitled “Gas Sensor Based on Protonic Conductive Membranes,” U.S. Pat. No. 5,618,493 entitled “Photon Absorbing Bioderived Organometallic Carbon Monoxide Sensors,” U.S. Pat. No. 5,650,054 entitled “Low Cost Room Temperature Electrochemical Carbon Monoxide and Toxic Gas Sensor with Humidity Compensation Based on Protonic Conductive Membranes,” U.S. Pat. No. 5,944,969 entitled “Electrochemical Sensor With A Non-Aqueous Electrolyte System,” U.S. Pat. No. 5,958,200 entitled “Electrochemical Gas Sensor,” U.S. Pat. No. 6,172,759 entitled “Target Gas Detection System with Rapidly Regenerating Optically Responding Sensors,” U.S. Pat. No. 6,200,443 entitled “Gas Sensor with a Diagnostic Device,” U.S. Pat. No. 6,936,147 entitled “Hybrid Film Type Sensor,” U.S. Pat. No. 6,948,352 entitled “Self-Calibrating Carbon Monoxide Detector and Method,” U.S. Pat. No. 7,077,938 entitled “Electrochemical Gas Sensor,” U.S. Pat. No. 7,022,213 entitled “Gas Sensor and Its Method of Manufacture,” U.S. Pat. No. 7,236,095 entitled “Solid State Sensor for Carbon Monoxide,” U.S. Pat. No. 7,279,081 entitled “Electrochemical Sensor,” U.S. Patent Publication No. 2005/0145494 entitled “Liquid Electrochemical Gas Sensor,” U.S. Patent Publication No. 2006/0091007 entitled “Gas Detecting Device with Self-Diagnosis for Electrochemical Gas Sensor,” U.S. Patent Publication No. 2006/0120924 entitled “Proton Conductor Gas Sensor,” and U.S. Patent Publication No. 2006/0196770 entitled “Liquid Electrochemical Gas Sensor,” all of which are hereby incorporated herein by reference in their entirety—including all references cited therein.
While the utilization of fuel cells and electrochemical sensors for use in gas, smoke, and/or fire detectors has become increasingly popular, sensor performance, cost, longevity, and/or configuration remains largely problematic.
Indeed, modern fuel cells and electrochemical sensors commonly use carbon supported noble metal catalysts, such as platinum at both the anode and the cathode. At the anode, platinum typically catalyses the oxidation of the fuel, such as hydrogen, methanol, carbon monoxide, etcetera. At the cathode, platinum typically catalyses the reduction of oxygen. For example, the chemical reactions that typically occur in an electrochemical carbon monoxide sensor are provided below:Anode: CO+H2O→CO2+2H++2e−Cathode: ½O2+2H++2e−→H2ONet: CO+½O2→CO2 
As is shown in FIG. 10, due to the identical nature of the catalyst electrodes, without biasing the electrodes, electric current generation (e−) can typically only occur if both the anode and the cathode are separated and sealed from each other to prevent gas crossover. The two electrodes are typically separated by a membrane that allows for diffusion of ions, such as protons (H+), and water (H2O). Gas crossover through the membrane is possible, but the diffusion rate of the sample gas is controlled so that the fuel gas (H2, CO, etcetera) is effectively scrubbed from the sample gas by the anode and only oxygen is allowed to crossover to the cathode.
When both the anode and the cathode comprise identical materials, in this case carbon supported platinum, there can be several problems. The first occurs when the gas diffusion is not controlled and the gases crossover from one electrode to the other. This can lead to degradation of the electrodes through peroxide formation or oxidation. In an electrochemical sensor, gas crossover results in reduced signal strength and polarization of the sensor which can potentially lead to sensor malfunction. Platinum is sensitive to poisoning from external contaminants, such as sulfur compounds, which reduces the electrical current being generated. Electrode sensitivity can also drop over time due to reduced surface area of the platinum particles caused by rearrangement and sintering. Additionally, amorphous carbon which is a common material used as a carbon support, (e.g., XC72 (Cabot Corporation) or Black Pearls (Cabot Corporation)) is susceptible to oxidation. The oxidation reaction can even be catalyzed by the very materials that the carbon is meant to support, such as Pt, Ru, Pd, etcetera. This oxidation can result in the presence of a background current in a sensor application, reduced electrical conductivity within the electrodes, and migration and aggregation of metal nanoparticles resulting in reduced power output or sensitivity. This is especially problematic for a sensor application in which long term drift and reduced sensitivity can be catastrophic. Finally, platinum is a noble metal that is rare and expensive.
It is therefore an object of the present invention, among other objects, to provide novel anodic and/or cathodic materials which replace conventional platinum based electrodes in fuel cells and electrochemical sensors. It is also an object of the present invention to provide novel device configurations which are enabled by the use of these novel materials for the anode and/or cathode.
These and other objects of the present invention will become apparent in light of the present specification, claims, and drawings.